(1) Field of the invention
The invention relates to an arrangement for applying a signal to a transmission line, in particular a telephone line which is symmetrically fed from a d.c. voltage source.
This field includes circuits for connecting telephone lines to automatic telephone exchanges, from where the telephone lines are fed with direct current and wherein speech signals are transferred from and to the telephone line.
(2) Description of the prior art
As shown in FIG. 1, a conventional, known connection circuit of this type comprises a transformer 1 having a primary winding 2 and a secondary winding which is divided into two portions 3 and 4. For alternating current, these two portions are interconnected by means of a capacitor 5. This capacitor blocks the direct current which is applied to the wires a and b of the telephone line TL by the central battery 8 by means of the resistors 6 and 7.
Speech signals which are applied to the terminals 9 and 10 of the primary winding will pass the circuit substantially without being attenuated when the inductances of the windings and the capacitance of capacitor 5 are of a sufficiently high value.
The telephone line TL is symmetrically fed with direct current through the resistors 6 and 7. The symmetry depends on the degree of equality of the resistors 6 and 7.
With a sufficiently high value of capacitor 5 the symmetry of the alternating current signal transfer from the terminals 9 and 10 to the telephone line will depend on the degree of equality of the windings 3 and 4.
For signals having equal phases on the a-wire and the b-wire, the so-called equal phase or "common-mode" signals, the connecting circuit offers the parallel arrangement of the supply resistors 6 and 7.